Method of anodizing valve metal derived anode bodies and electrolyte therefore

ABSTRACT

An electrolyte solution for anodizing a metal and a capacitor comprising the anodized metal. The electrolyte comprises more than about 5%, by weight, and less than about 30%, by weight, water; about 0.1 to 20%, by weight, ionogen and an aprotic polar solvent. The ionogen comprises phosphoric acid and an alkanol amine in an amount, and ratio, sufficient to maintain a pH of about 4 to about 9.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an electrolyte for anodizingvalve metals and a method utilizing the same.

[0002] With the introduction of tantalum “wet slug” capacitors in the1940's, the use of aqueous phosphoric acid anodizing solutions, operatedat 80-90° C., rapidly became the industry standard. These solutions areparticularly suitable for anodizing tantalum powder metallurgy anodebodies, which are the basis of these capacitors. Phosphoric acidanodizing provides superior properties to the dielectric films. It isknown that the presence of phosphate in the anodic oxide dielectric,from the anodizing solution, greatly reduces the mobility of oxygenthrough the oxide. The decreased mobility minimizes oxygen migrationinto the substrate resulting in a more stable dielectric than thatachieved in the absence of phosphate. By about 1960, aqueous ethyleneglycol solutions of dilute phosphoric acid containing 10-60% ethyleneglycol had replaced aqueous phosphoric acid for the anodizing oftantalum powder metallurgy capacitor anode bodies, particularly athigher anodizing voltages. Enhancements in dielectric properties areobtained with ethylene glycol present in the anodizing solution.

[0003] The action of the ethylene glycol in producing superiordielectric quality appears to be quite complex. The presence of ethyleneglycol may modify the boiling point of the electrolyte, the resistivityversus temperature response of the electrolyte, as well as the ultimatesparking voltage of the electrolyte. Secondary ion mass spectroscopy (or“SIMS” analysis) of anodic oxide films grown on tantalum in aqueousethylene glycol/dilute phosphoric acid indicates the presence of carbonin these films. The difficulties encountered in the analysis of anodicoxide films have so far prevented the identification of the carbonspecies present in the glycol/phosphate formed films. The carbon speciesmay be present as a glycol phosphate ester, a glycol oxidation productsuch as oxalate, formate or carbonate, or some as yet unanticipatedspecies. The great stability of the incorporated carbon species duringthe heat treatment at temperatures of from about 250° C. to about 450°C. strongly suggests that the incorporated species is carbonate. A moredefinitive answer awaits more sensitive methods of oxide film analysis.A second anodizing step typically follows the heat treatment to furtherenhance oxide stability.

[0004] U.S. Pat. No. 5,716,511 describes a method, and electrolyte, forproducing anodic films on tantalum and other valve metal bodies for thepurpose of minimizing the number of flaws in the resulting dielectricfilms. These electrolytes are usually employed at temperatures belowabout 50° C. The minimum water content of the electrolyte is necessaryfor reasonable uniform anodic oxide formation throughout the bulk oftantalum powder metallurgy compacts of about 30%. This is consistentwith the ethylene glycol containing electrolytes. Below approximately30% water content, the anodizing is found to take place largely near theouter surfaces of the anode bodies with the interiors of the anodebodies remaining largely unanodized unless extremely long hold times atvoltage (+48 hours) are employed.

[0005] U.S. Pat. No. 6,480,371 describes the use of anodizing solutionscontaining akanolamines and phosphoric acid for the purpose ofmaximizing the possible anodizing voltage and minimizing the depositionof polyphosphates within the anode bodies. The use ofalkanolamine/phosphoric acid mixtures in combination with the aqueouspolyethylene glycol dimethyl ethers, as described in U.S. Pat. No.5,716,511, has been found to yield particularly stable dielectric films.The electrolytes resulting from the combination of the solvents of U.S.Pat. No. 5,716,511 and the ionogens of U.S. Pat. No. 6,480,371 stillrequire a minimum water content of approximately 30% water for proper,uniform, anodizing within the bodies of powder metallurgy anodes.

[0006] United Kingdom Patent Application No. GB 2,168,383 describes amethod for anodizing a wide variety of valve metals using polar, aproticsolvent solutions of phosphoric acid, or electrolyte and water-solublephosphates, containing less than 2% water. For many valve metals, theseelectrolytes give the best results when operated below about 30° C. andat a current density of about 1 milliampere/cm² or less. U.S. Pat. No.5,185,075 extends the water content of the solutions described in GB2,168,383, and the operating temperature to 50° C. or less for theanodizing of 99.997% pure titanium. Neither the solvents described in GB2,168,383 or the variation described in U.S. Pat. No. 5,185,075 aregenerally suited for use in the anodizing of powder metallurgy anodes.At the low water content of the electrolytes of these patents theinternal portions of the anodes are not uniformly anodized by phosphoricacid/aprotic polar solvent solutions.

[0007] It is known that certain polar aprotic solvents have a tendencyto form complexes with protonated amines. The complexes yieldnon-aqueous solutions which are more electrically conductive thansolutions containing the same amine and acid but in a non-aqueoussolvent which does not form strong complexes with protonated amines.Examples of polar, aprotic solvents which form complexes with protonatedamines include N-alkyl amides, such as dimethyl formamide. Examples ofpolar, aprotic solvents which have a much lower tendency to formcomplexes with protonated amines include N-methyl-2-pyrrolidone,N-ethyl-2-pyrrolidone, 4-butylrolactone, propylene carbonate,tetramethyl urea, and sulfolane (tetramethylene sulfone). One member ofthe group of solvents which form complexes with protonated amines isdimethyl sulfoxide. The tendency of dimethyl sulfoxide to form complexeswith protonated amines is sufficiently great that dimethyl sulfoxide hasbeen employed as a component of working or fill electrolytes forelectrolytic capacitors. U.S. Pat. No. 4,812,951 describes the effectsof substituting 25% dimethyl sulfoxide for an equivalent amount ofprimary capacitor solvent. The result is a reduction in the lowtemperature resistivity and a minimization of the change in resistivityof the electrolyte with changing temperatures.

BRIEF SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide anelectrolyte for anodizing valve metals and a method utilizing same.

[0009] It is another object of the present invention to provide anaqueous electrolyte suitable for anodizing a valve metal withoutminimizing anodization of the anode interior.

[0010] A particular feature is the ability to anodize at higher pHwithout decreasing the effectiveness while still incorporating phosphatein the oxide layer.

[0011] Another particular feature is the ability to incorporatephosphate in the oxide layer without excessive polyphosphatedepositions.

[0012] These and other advantages, as would be realized to one ofordinary skill in the art, are provided in an electrolyte solution foranodizing a metal and a capacitor comprising the anodized metal. Theelectrolyte comprises more than about 5%, by weight, and less than about30%, by weight, water; about 0.1 to 20%, by weight, ionogen and anaprotic polar solvent. The ionogen comprises phosphoric acid and analkanol amine in an amount, and ratio, sufficient to maintain a pH ofabout 4 to about 9.

[0013] Another embodiment is provided in a method for anodizing an anodecomprising the steps of:

[0014] a) providing a valve metal;

[0015] b) placing the valve metal in an electrolyte comprising:

[0016] more than about 5%, by weight, and less than about 30%, byweight, water;

[0017] about 0.1 to 20%, by weight, ionogen comprising:

[0018] phosphoric acid; and

[0019] an alkanol amine wherein the amount of ionogen is sufficient tomaintain a pH of about 4 to about 9; and

[0020] an aprotic polar solvent; and

[0021] c) subjecting the valve metal to an anodizing voltage.

[0022] Yet another embodiment is provided in A capacitor formed by theprocess of:

[0023] a) providing a valve metal;

[0024] b) placing the valve metal in an electrolyte comprising:

[0025] more than about 5%, by weight, and less than about 30%, byweight, water;

[0026] about 0.1 to 20%, by weight, ionogen comprising:

[0027] phosphoric acid; and

[0028] an alkanol amine wherein the amount of ionogen is sufficient tomaintain a pH of about 4 to about 9; and

[0029] an aprotic polar solvent;

[0030] c) subjecting the valve metal to an anodizing voltage to form ananodized anode; and

[0031] d) forming a capacitor with the anodized anode.

DETAILED DESCRIPTION OF THE INVENTION

[0032] The inventors of the present application have developed, throughdiligent research, an aqueous anodizing solution suitable for use at apH of 4-9 comprising alkanol amine and an aprotic polar solvent. Theinventors have also developed an improved method for anodizing an anodeutilizing the inventive anodizing solution.

[0033] For certain anodizing applications it is desirable to employanodizing solutions having a water content below approximately 30%. Thisis particularly the case with higher voltage anodizing of niobium orniobium suboxide powder metallurgy anodes or the higher voltageanodizing of powder metallurgy tantalum anodes fabricated from very finetantalum powders. The higher water amount is required for uniform anodicoxide growth within the interstices of powder metallurgy anodes with thetraditional organic solvents used in anodizing electrolytes, such asethylene glycol, diethylene glycol, polyethylene glycol 300,tetraethylene glycol dimethyl ether, etc. For the anodizing applicationsmentioned above, it is generally desirable to conduct the anodizing attemperatures below about 50° C. in order to minimize reactivity and flawinitiation in the anodic oxide films. As stated above, it is verydifficult to obtain uniform anodic oxide growth within powder metallurgyanode bodies with an electrolyte water content below about 30% water,this tendency to form non-uniform anodic oxide films within powdermetallurgy anode bodies is greatly aggravated at lower anodizingtemperatures. The undesirable deposition of polyphosphates within theinterstices of powder metallurgy anode bodies is also worsened by theuse of low water content electrolytes containing phosphoric acid as theionogen and operated at temperatures below about 80° C.

[0034] We have found that organic solvent solutions of phosphoric acidmay be adjusted to higher pH, i.e. the more nearly neutral pH range of4-9 which is useful for anodizing acid-sensitive substrates, via theaddition of certain amines to the solution of phosphoric acid in organicsolvents without precipitation of the amine phosphate salts. Morepreferably the pH is about 6 to about 8.

[0035] The alkanol amines which we have found to give the best resultsinclude monoethanol amine, diethanol amine, triethanol amine, ethyldiethanolamine, diethyl ethanolamine, dimethyl ethanolamine and dimethylethoxy ethanolamine (dimethyl amino ethoxy ethanol). These solventsprovide resistance to precipitation at high concentrations withphosphoric acid in organic solvent solutions. Most preferred aredimethyl ethanolamine and dimethyl ethoxy ethanolamine.

[0036] The phosphoric acid and alkanol amine are taken together torepresent the ionogen. The ratio of phosphoric acid amount to alkanolamine amount is determined based on the pH to be achieved. The amount ofalkanol amine is preferably increased for lower resistivity within theconstraints of pH. Preferably, the total amount of ionogen represents atleast about 0.1 to about 20%, by weight, of the total electrolyte.Preferably, the ionogen represents about 0.1 to about 15%, by weight, ofthe total electrolyte. In a particularly preferred embodiment thephosphoric acid is present in an amount of about 0.5 to about 5%, byweight, based on the total electrolyte. In a particularly preferredembodiment the alkanol amine is present in an amount of about 0.5 toabout 7%, by weight, based on the total electrolyte.

[0037] Unfortunately, solutions of phosphoric acid and alkanol amines,such as dimethyl ethanolamine or dimethyl ethoxy ethanolamine, in mostorganic solvents are not useful for anodizing powder metallurgy anodecompacts unless a minimum of about 30% water is present in the solution.Below about 30% water concentration non-uniform oxide growth occurs.

[0038] We have found that polar aprotic solvents which have a strongtendency to form complexes with protonated amines, for example, lowerN-alkyl amides (such as dimethyl formamide), and especially dimethylsulfoxide, may be used as the solvent portion of anodizing electrolytescontaining phosphoric acid in combination with alkanol amines as theionogen. The ionogens are more ionized in these electrolytes. Increasedionization results in greater conductivity within the interstices ofpowder metallurgy anodes. This greater ionization of theamine/phosphoric acid ionogen within the interstices of powdermetallurgy, or other porous anode bodies, anodized in the electrolyte ofthe present invention makes possible a reduction in the water content ofthe electrolyte to levels below the 30% minimum necessary withconventional anodizing solvents, such as ethylene glycol. We have foundthat the water content of the electrolyte should be greater than about5% in order to insure uniform oxide formation within the interstices ofporous anode bodies such as powder metallurgy anode bodies.

[0039] For the purpose of the present invention, the water content ofthe electrolyte refers to the free or uncombined water, not to thecombined water content of the phosphoric acid content of theelectrolyte. Phosphoric acid may be considered to be a chemicalcombination of phosphorous oxide and water. By way of example, oneequivalent of P₂O₅ may be combined with 3 equivalents of water to yield2 equivalents of phosphoric acid, H₃PO₄. The water contained in thephosphoric acid molecules is not considered as solution water for thepurposes of this invention.

[0040] The anode is a valve metal preferably chosen from titanium,tungsten, chromium, aluminium, zirconium, hafnium, zinc, vanadium,niobium, tantalum, bismuth, antimony and mixtures, alloys and metallicglass compositions thereof. Tantalum is the most preferred anode.

[0041] The cathode is a conductive metal provided with a semiconductiveor pseudoconductive coating. The coating can be an oxide, nitride,carbide or carbon nitride. Suitable cathode metals include tantalum,titanium, nickel, iridium, platinum, palladium, gold, silver, cobalt,molybdenum, ruthenium, manganese, tungsten, iron, zirconium, hafnium,rhodium, vanadium, osmium and niobium. A particularly preferred cathodeelectrode comprises a porous ruthenium oxide film provided on a titaniumsubstrate.

EXAMPLE

[0042] An electrolyte solution was prepared containing: 180 grams ofdimethyl sulfoxide; 30 grams of de-ionized water; 10 grams of 85%phosphoric acid comprising 1.5 grams of water and 10 grams of dimethylethanolamine. The pH was measured with Hydrion paper to be about 6-7.The resistivity, at 1 kHz, was measured to be about 1,090 ohm-cm @ 29°C. and the water content was determined to be about 13.7%, by weight.

[0043] This electrolyte solution was placed in a 250 ml stainless steelbeaker, which served as the cathode connection.

[0044] An anode was fabricated from H. C. Starck QR-12 tantalum powder,weighing 1.91 grams and from a group of anodes having a CV product of4,200 microcoulombs (i.e., microfarad-volts) after anodizing to 150volts in dilute phosphoric acid at 80° C., was anodized to 50 volts inthe above solution at approximately 25° C., at a current of 20milliamperes. The anode was held at voltage for about 7.5 hours. Theanode was then rinsed, dried and the capacitance was measured in anaqueous dibasic potassium phosphate solution.

[0045] The capacitance was found to be 100.5 microfarads. One maycalculate the 80° C. equivalent of the 50 volt anodizing voltage at 25°C. using the Torrisi relation V₁T₁=V₂T₂, wherein V is the anodizingvoltage and T is the temperature (Kelvin) of the anodizing electrolyte.Thus for the present example, 50 volts at 25° C. is equivalent to 42volts at 80° C.

[0046] The CV for the anode after anodizing in an electrolyte of thepresent invention is then calculated as 42 volts multiplied by 101.5microfarads the product of which is 4263 microfarad-volts. This is inclose agreement with the 4,200 CV found for this lot of anodes viaphosphoric acid anodization. The anode was uniformily anodizedinternally.

EXAMPLE 2

[0047] An additional anode from the same lot used in Example 1 wasanodized in the same electrolyte as in Example 1. In this case, theanode was anodized to 125 volts at 20 milliamperes and at 25° C. Theanode was held at voltage for about 19 hours. The anode was then rinsed,dried, and the capacitance was measured as in Example 1. The capacitancewas found to be 38.95 microfarads.

[0048] Again, using the Torrisi relation, 125 volts at 25° C. isequivalent to 105.9 volts at 80° C. The CV product is then calculated tobe the multiplicative product of 38.95 microfarads and 105.9 volts whichis 4125 CV. This is in good agreement with the 4,200 CV found for thisanode lot formed in dilute phosphoric acid. The anode was uniformlyanodized internally.

EXAMPLE 3

[0049] A higher voltage capability electrolyte was formulated asfollows: 900 ml dimethyl sulfoxide; 100 ml deionized water; 13.0 gramsof 85% phosphoric acid comprising 2 grams of water; and 10.5 grams ofdimethyl ethanolamine. The pH was measured to be about 6 with Hydrionpaper. The resistivity, at 1 kHz, was determined to be about 2,400ohm-cm @ 23° C. and the water content was about 10%, by weight.

[0050] An anode from the same lot as used in Examples 1 and 2 wasanodized to 240 volts at 25° C. The anode was then held at voltage forapproximately 21 hours. The anode was then rinsed, and the capacitancewas found to be 20.2 microfarads. Again, using the Torrisi relation, 240volts at 25° C. is equivalent to about 203.4 volts.

[0051] The CV is then found as the multiplicative product of 20.2microfarads and 203.4 volts to be about 4,109 microfarad-volts. This isin excellent agreement with the 4,200 CV for this anode lot with dilute,80° C. phosphoric acid anodized to 150 volts.

[0052] The CV products at various voltages for Examples 1-3 are providedin Table 1. TABLE 1 80° C. Voltage Equivalent Capacitance CV Product  42 volts 101.5 μF 4,263 μF-V 105.9 volts 38.95 μF 4,125 μF-V 203.4volts  20.2 μF 4,109 μF-V

EXAMPLE 4

[0053] To illustrate the utility of the lower N-alkyl amides anelectrolyte was prepared comprising 180 grams of dimethyl formamide; 20grams of de-ionized water, 10 grams of 85% phosphoric acid comprising1.5 grams of water; and 13.3 grams of dimethyl ethoxy ethanolamine. ThepH was measured at about 6-7 with Hydrion paper. The resistivity, at 1kHz, was about 1,250 ohm-cm @ 32° C. and the water content was about9.6%, by weight.

[0054] An anode from the same lot as used for Examples 1-3 was anodizedat 50 volts at a current of 20 milliamperes and at a temperature of 21°C. The anode was held at voltage for a period of nearly 5 hours. Theanode was then rinsed in de-ionized water, dried and the capacitance wasmeasured in aqueous dibasic potassium phosphate solution. The measuredcapacitance value was 100.3 microfarads. Using the Torrisi relation, 50volts at 21° C. is equivalent to about 41.6 volts at 80° C. The CV isthen found to be the multiplicative product of 100.3 microfarads and41.6 volts or about 4,172 microfarad-volts. Again, this is in closeagreement with the 4,200 μF-V per anode CV value for this lot asdetermined by anodizing 150 volts at 80° C. in dilute phosphoric acid.

[0055] Thus the inventors have found that the strong tendency of lowerN-alkyl amides and dimethyl sulfoxide to form complexes with protonatedamines may be exploited to prepare anodizing electrolytes containingabove about 5% water and below about 30% water, useful for the anodizingof porous anode bodies, at temperatures below about 50° C.

[0056] The invention has been described with particular emphasis on thepreferred embodiments. It would be realized from the teachings hereinthat other embodiments, alterations, and configurations could beemployed without departing from the scope of the invention which is morespecifically set forth in the claims which are appended hereto.

Claimed is:
 1. An electrolyte solution for anodizing a metal comprising:more than about 5%, by weight, and less than about 30%, by weight,water; about 0.1 to 20%, by weight, ionogen comprising: phosphoric acid;and an alkanol amine wherein the amount of ionogen is sufficient tomaintain a pH of about 4 to about 9; and an aprotic polar solvent. 2.The electrolyte of claim 1 wherein said alkanol amine is selected frommonoethanol amine, diethanol amine, triethanol amine, ethyldiethanolamine, diethyl ethanolamine, dimethyl ethanolamine and dimethylethoxy ethanolamine.
 3. The electrolyte of claim 2 wherein said alkanolamine is selected from dimethyl ethanolamine and dimethyl ethoxyethanolamine.
 4. The electrolyte of claim 1 wherein said aprotic polarsolvent comprises an N-alkyl amide.
 5. The electrolyte of claim 4wherein said N-alkyl amide is selected from dimethyl formamide anddimethyl acetamide.
 6. The electrolyte of claim 1 wherein said aproticpolar solvent comprises dimethyl sulfoxide.
 7. The electrolyte of claimI comprising about 0.1 to about 15%, by weight, said ionogen.
 8. Theelectrolyte of claim 7 comprising about 0.5 to about 5%, by weight,phosphoric acid.
 9. The electrolyte of claim 7 comprising about 0.5 toabout 7%, by weight, alkanol amine.
 10. An anode anodized by theelectrolyte of claim
 1. 11. An anode of claim 10 comprising a valvemetal selected from titanium, tungsten, chromium, aluminium, zirconium,hafnium, zinc, vanadium, niobium, tantalum, bismuth, antimony andmixtures, alloys and metallic glass compositions thereof.
 12. The anodeof claim 11 wherein said anode comprises tantalum.
 13. A capacitorcomprising the anode of claim
 12. 14. A method for anodizing an anodecomprising the steps of: providing a valve metal; placing said valvemetal in an electrolyte comprising: more than about 5%, by weight, andless than about 30%, by weight, water; about 0.1 to 20%, by weight,ionogen comprising: phosphoric acid; and an alkanol amine wherein theamount of ionogen is sufficient to maintain a pH of about 4 to about 9;and an aprotic polar solvent; and subjecting said valve metal to ananodizing voltage.
 15. The method for anodizing an anode of claim 14wherein said alkanol amine is selected from monoethanol amine, diethanolamine, triethanol amine, ethyl diethanolamine, diethyl ethanolamine,dimethyl ethanolamine and dimethyl ethoxy ethanolamine.
 16. The methodfor anodizing an anode of claim 15 wherein said alkanol amine isselected from dimethyl ethanolamine and dimethyl ethoxy ethanolamine.17. The method for anodizing an anode of claim 14 wherein said aproticpolar solvent comprises an N-alkyl amide.
 18. The method for anodizingan anode of claim 17 wherein said N-alkyl amide is selected from a groupconsisting of dimethyl formamide and dimethyl acetamide.
 19. The methodfor anodizing an anode of claim 14 wherein said aprotic polar solventcomprises dimethyl sulfoxide.
 20. The method for anodizing an anode ofclaim 14 comprising about 0.1 to about 15%, by weight, said ionogen. 21.The method for anodizing an anode of claim 20 comprising about 0.5 toabout 5%, by weight, said phosphoric acid.
 22. The method for anodizingan anode of claim 20 comprising about 0.5 to about 7%, by weight, saidalkanol amine.
 23. An anode anodized by the method of claim
 14. 24. Theanode of claim 23 wherein said valve metal is selected from titanium,tungsten, chromium, aluminium, zirconium, hafiium, zinc, vanadium,niobium, tantalum, bismuth, antimony and mixtures, alloys and metallicglass compositions thereof.
 25. The anode of claim 24 wherein said valvemetal comprises tantalum.
 26. A capacitor comprising the anode of claim25.
 27. A capacitor formed by the process of: providing a valve metal;placing said valve metal in an electrolyte comprising: more than about5%, by weight, and less than about 30%, by weight, water; about 0.1 to20%, by weight, ionogen comprising: phosphoric acid; and an alkanolamine wherein the amount of ionogen is sufficient to maintain a pH ofabout 4 to about 9; and an aprotic polar solvent; and subjecting saidvalve metal to an anodizing voltage to form an anodized anode; forming acapacitor with said anodized anode.
 28. The capacitor of claim 27wherein said alkanol amine is selected from monoethanol amine, diethanolamine, triethanol amine, ethyl diethanolamine, diethyl ethanolamine,dimethyl ethanolamine and dimethyl ethoxy ethanolamine.
 29. Thecapacitor of claim 28 wherein said alkanol amine is selected fromdimethyl ethanolamine and dimethyl ethoxy ethanolamine.
 30. Thecapacitor of claim 27 wherein said aprotic polar solvent comprises anN-alkyl amide.
 31. The capacitor of claim 30 wherein said N-alkyl amideis selected from dimethyl formamide and dimethyl acetamide.
 32. Thecapacitor of claim 31 wherein said aprotic polar solvent comprisesdimethyl sulfoxide.
 33. The capacitor of claim 27 wherein saidelectrolyte comprises about 0.1 to about 15%, by weight, ionogen. 34.The capacitor of claim 33 wherein said electrolyte comprises about 0.5to about 5%, by weight, phosphoric acid.
 35. The capacitor of claim 33wherein said electrolyte comprises about 0.5 to about 7%, by weight,said alkanol amine.
 36. The capacitor of claim 27 wherein said valvemetal is selected from titanium, tungsten, chromium, aluminium,zirconium, hafnium, zinc, vanadium, niobium, tantalum, bismuth, antimonyand mixtures, alloys and metallic glasses thereof.
 37. The capacitor ofclaim 36 wherein said valve metal comprises tantalum.